1. Technical Field
This invention relates to apparatus and methods for analyzing the composition of formation fluids, and more particularly to apparatus and methods for using near infrared spectral analysis to determine the quantities of gas, water and various types of oils in a formation fluid.
2. Background Information
As seen in FIG. 1, several different interactions may occur when light strikes a sample. Typically, if the sample is fluid, some light is reflected at the boundary of the sample while the rest of the light enters the sample. Inside the sample, light is scattered by molecular excitations (Raman scattering) and by collective modes of the medium (e.g. Rayleigh scattering). In general, only a very small fraction of the light is scattered per centimeter of the path by the Raman and Rayleigh scattering processes.
If more than one phase is present in the sample, light is elastically scattered by reflection and refraction at the boundaries between the phases. This scattering process can be quite strong as light may be scattered many times in less than one centimeter of the path. Light which is not scattered or which is scattered but emerges from the sample travelling in a direction nearly parallel to and in the same direction as the incident light is generally referred to as "transmitted". Light which emerges travelling in other directions is referred to as "scattered", while light which emerges travelling in a direction nearly opposite to the incident light is referred to as "backscattered".
Regardless of scattering, some light is absorbed by the sample. The fraction of incident light absorbed per unit of pathlength in the sample depends on the composition of the sample and on the wavelength of the light. Thus, the amount of absorption as a function of wavelength, hereinafter referred to as the "absorption spectrum", an indicator of the composition of the sample. In the wavelength range of 0.3 to 2.5 microns, which is the range of primary interest according to this invention, there are two important absorption mechanisms in borehole fluids. In the near infrared region (1 to 2.5 microns), absorption results primarily from the excitation of overtones of molecular vibrations involving hydrogen ions in the borehole fluids. In the near ultraviolet, visible, and very near infrared regions (covering wavelengths of 0.3 to 1 micron), absorption results primarily from excitation of electronic transitions in large molecules in the borehole fluids such as asphaltenes, resins, and porphyrins.
In the past, techniques have been known for the qualitative and quantitative analysis of gas, liquid, and solid samples. Methods and apparatus for accomplishing the same are disclosed in U.S. Pat. No. 4,620,284 to R. P. Schnell where a helium-neon laser is used to provide photons of a 0.633 micron wave length which are directed at a sample. The resulting Raman spectrum which comprises scattered light at different wavelengths than the incident light is then measured, and the measured spectrum is compared with previously obtained reference spectra of a plurality of substances. The provided technique is applied to monitoring fluid flowing through a pipeline in an oil refinery. In U.S. Pat. No. 4,609,821 to C. F. Summers, especially prepared rock cuttings containing at least oil from an oil-based mud are excited with UV radiation with a 0.26 micron wave length. Instead of measuring the Raman spectrum as is done in the aforementioned Schnell patent, in accord with the Summers disclosure, the frequency and intensity of the resulting excited waves (fluorescence) which are at a longer wavelength than the incident radiation are detected and measured. By comparing the fluorescent spectral profile of the detected waves with similar profiles of the oil used in the oil-based mud, a determination is made as to whether formation oil is also found in the rock cuttings.
While the Summers and Schnell disclosures may be useful in certain limited areas, it will be appreciated that they suffer from various drawbacks. For example, the use of laser equipment in Schnell severely restricts the environment in which the apparatus may be used, as lasers are not typically suited to harsh temperature and/or pressure situations (e.g. a borehole environment). Also, the use of the Raman spectrum in Schnell imposes the requirement of equipment which can detect with very high resolution the low intensity scattered signals. The use by Summers of light having a 0.26 micron wavelength severely limits the investigation of the sample to a sample of nominal thickness. In fact, the Summers patent requires that the sample be diluted with solvents before investigation. Thus, the Summers patent, while enabling a determination of whether the mud contains formation oil, does not permit an analysis of formation fluids in situ. Finally, the Summers method has no sensitivity to water.
Those skilled in the art will appreciate that the ability to conduct an analysis of formation fluids downhole is extremely desirable. A first advantage would be the ability to distinguish between formation fluids and mud filtrate, thereby permitting a fluid extraction tool to retain only fluids of interest for return to the formation surface. A second advantage is in the production phase, where a determination of the fluid type (i.e. water, oil, or gas) entering the well from the formations can be made immediately downhole.